Most types of glass, and in particular plate glass and container glass, are manufactured by melting of raw materials in large melting furnaces producing a few tens to a few hundred metric tons of glass per day and per unit. The fuel used in such furnaces is generally natural gas or fuel oil, although other fuels can also be used. Certain furnaces also use electricity to increase production (electric boosting). High temperature furnaces (typically 1,500° C., but sometimes higher) are necessary for the melting. Optimal furnace temperature conditions are obtained by pre-heating the combustion air (typically up to 1,000° C., but sometimes higher). The heat required for pre-heating the combustion air is transmitted by the waste gases, which is generally effected by using reversible regenerators. This approach enables one to obtain a high degree of thermal efficiency combined with high melting rates. Several types of melting furnaces exist, including:
Cross-fired furnaces: in these furnaces, which have a melting surface area generally greater than 70 me and which operate with a reversal of the direction of the flame approximately every 20-30 min, the heat contained in the waste gases is recovered in regenerators made up of stacks of refractory bricks. The cold combustion air is pre-heated during its passage through the regenerators (rising air), while the hot waste gases leaving the furnace are used to re-heat other regenerators (descending waste gases). These furnaces, with an output sometimes greater than 600 t/day and which are used for manufacturing plate glass and container glass, are great energy consumers. The diagram of cross-fired furnace operation is presented in FIG. 1.
End-fired furnaces: in these furnaces, the flame (sometimes called a horseshoe flame) describes a loop. These furnaces operate with recovery of the heat of the waste gases by stacks forming regenerators which deliver it to the combustion air. The diagram of the operation of this type of furnace is presented in FIG. 2.
The fuel is injected into the furnace into or near the air stream leaving the regenerator. The burners are designed to produce high temperature flames with good radiative qualities so as to obtain an efficient heat transfer. A certain number of options exist for producing the comburant/fuel mixture. The names of these techniques show how the fuel is introduced. The most frequently encountered configurations are the following:                “Under port”: under the air stream,        “Over port”: above the air stream,        “Side port”: beside the air stream,        “Through port”: through the air stream.        
The choice among these different injection methods is made so as to obtain a suitable result for the configuration of the air streams and of the type of melting furnace used, and as a function of constraints connected with fuel supply (example: available gas pressure for a furnace supplied with natural gas) or with the nature of the fuel.
Although such combustion methods are very efficient in terms of furnace operation, they induce adverse effects such as the production of very high levels of nitrogen oxides (subsequently called: NOx), one of the most regulated air pollutants. In the majority of industrialized countries, limits (in terms of concentration and flow rate) are imposed on large capacity glass making furnaces in order to reduce NOx emissions. Furthermore, regulation is becoming more drastic each year.
In high temperature furnaces as in glass melting furnaces, the main avenue of NOx formation is the “thermal” avenue in which the NOx are produced in zones of the furnace where flame temperatures are greater than 1,600° C. Beyond this threshold, the formation of NOx increases exponentially with the flame temperature. Unfortunately, the combustion techniques generally used in melting furnaces for creating very radiative flames such as those mentioned in the preceding induce high flame temperatures (with maxima greater than 2,000° C.) and have the consequence of generating NOx emissions much higher than those accepted in numerous countries of the world.
Furthermore, one of the consequences of conventional combustion methods is that there is little heat released by combustion in the major part of the volume of the furnace, since in effect the combustion products surrounding the flame gradually cool in giving up their heat to the glass bath.
Over time, the waste gases become less efficient in transferring heat to the glass bath by radiation. The transfer of heat by radiation from the flame to the glass bath can be increased to a significant degree if a way is found to increase the temperature of the combustion products still present in the melting chamber.
Several techniques exist enabling reduction of regenerative melting furnace NOx emissions. Among these can be distinguished primary methods (in which reduction occurs during combustion), secondary methods (in which reduction occurs by treatment of the combustion products at the furnace outlet) and intermediate methods in which the treatment occurs at the location of the outlet of the melting chamber to the regenerators (the Pilkington 3R process or re-burning).
The methods which can be used are the following:
Primary method—“Low-NOx” burners: There are several types of “low NOx” burners on the market, that is to say burners which enable reducing the NOx emissions even when used alone. However, their performances do not always enable obtaining the necessary reduction levels for compliance with European regulations or those in force in other countries around the world. More particularly, the following types of burners are encountered:
Double impulse burners—These burners produce a low gas speed at the root of the flame so as to reduce the temperature of the flame in the zone where the majority of the NOx is generated. The burners also increase the luminosity of the flame, which promotes a lowering of the temperature of the flame front by increasing the radiative transfer of heat to the glass bath.
Injection of enveloping gas or “Shrouded Gas Injection”—With this technique, gaseous fuel is injected at low speed above the “underport” burners in order to block the comburant flows and to delay mixing of the gas at high speed coming from the “underport” burners with the air streams, thus reducing temperatures at the root of the flame.
Ultra-low speed injection of the gas—Injections of fuel gas at very low speeds (less than 30 m/s) are used with special burners cooled by water circulation in order to minimize the local temperature of the flame and to increase its luminosity. The efficiency of this type of burner in terms of NOx reduction depends greatly on the design of the furnace.
Primary method—Staging: This technique uses conventional burners for injection of the fuel and reduction of the flow of combustion air through the air stream in order to create conditions of excess fuel and to introduce the rest of the comburant in another location of the furnace in order to complete oxidation of the fuel. This method, which can drastically reduce NOx emissions, is nevertheless difficult to implement and expensive to use since it requires pure oxygen or ducts for introducing air at temperatures higher than 1,000° C. in order to be thermally efficient (staging of the comburant in cold air would induce a reduction of energy efficiency). Examples of this staging technique are:
Air staging: Diverting the hot combustion air coming from the regenerators by using an ejector towards the combustion chamber in the direction of the waste gases so as to produce complete combustion. This method requires the use of heat-insulated ducts and cold air for directing the ejector, hence a loss of thermal efficiency. This technique has only been used on end-fired furnaces, and mainly in Germany.
Oxygen-enriched air staging or OEAS (for Oxygen Enriched Air Staging): The combustion air entering the air stream is introduced with an insufficient flow for complete combustion in order to create sub-stoichiometric conditions, and pure oxygen or oxygen enriched air is injected at the rear of the furnace towards the flow of waste gases so as to complete combustion in the recirculation zone of the furnace. The OEAS injectors are generally installed in underport position separately from the burners. This technique has been applied successfully in end-fired furnaces and in cross-fired furnaces, and mainly in the United States.
Among the various staging technologies, the patent WO 97/36134 discloses a device with line burners. This device makes it possible to stage the fuel within the air stream. The fuel supplied to the furnace is divided in two, and a portion is injected upstream of the burner directly into the hot combustion air. This methodology does not use an injection of fuel directly into the combustion chamber as in the present invention. The technique uses an injection of fuel but always coupled with an injection of air.
Primary Method—Rich Operating Conditions:
This technique reduces the NOx emissions by injecting additional fuel into the combustion chamber so as to create a “reducing atmosphere” in the combustion chamber. The reducing atmosphere converts the NOx formed in the flame into nitrogen and oxygen. In this technique, the NOx produced in the high temperature flame front are reduced in a second step.
In effect, as shown, for example, by the document JP-A-08208240, additional fuel introduced by injectors situated on the wall supporting the burner, on the side wall or facing the burner, is added to the original fuel supplying the burner or burners. However, according to this method, while making possible considerable NOx reductions in the combustion chamber, it is necessary to provide additional combustion air after the exit from the combustion chamber. Not only does this method require additional consumption of fuel, but the additional fuel is not burned in the combustion chamber and therefore does not participate in the melting of the glass.
This process uses an over-consumption of fuel in order to reduce the NOx, and its application can lead to an increase of the temperatures in the regenerators and in time to degradation of the regenerators.
Secondary method—Treatment of the waste gases: A major portion of the NOx is treated at the outlet of the furnace by the use of chemical reduction processes. Such processes require the use of reducing agents such as ammonia or hydrocarbon-containing combustion residues with or without the presence of catalysts. Although capable of achieving the NOx level reductions set by regulations, these processes are very expensive to install and operate, and in the case of processes based on hydrocarbons such as the 3R process or system explained hereafter, a 5-15% increase of the fuel consumption is observed. Examples of this technique are given below:
3R process (Reaction and Reduction in Regenerators; patented process of the Pilkington company)—In this technique, the gas is injected at the chamber roof so as to consume any excess air and to produce reducing conditions in the regenerators situated at the outlet of the furnace, resulting in the conversion of the NOx into nitrogen and oxygen. Since an excess of gas must be used, it is consumed in the lower part of the regenerators where the air is infiltrated or injected. The additional heat generated is often recovered by boilers. In order to minimize the quantity of gas necessary for the 3R system, it is common to operate the furnace with the lowest possible excess air. This technology enables achieving the NOx reduction levels imposed by the current regulations, and even to exceed them. Generally, 5-15% of the total fuel consumption of the furnace is necessary for implementation of the 3R process. The reducing atmosphere in the regenerators is often the cause of problems with the refractory material composing them.
Selective catalytic reduction or SCR (Selective Catalytic Reduction)—This method uses a platinum catalyst for reacting the NOx with ammonia or urea so as to reduce the NOx into N2 and water. The process has to take place at a specific temperature and requires precise control of the ammonia in order to avoid accidental pollution-generating discharges. Since this reaction occurs on the surface, large surface areas of catalyst are necessary, involving relatively large installations. The chemical process is relatively complex and demanding in terms of control and maintenance. Very high NOx reduction levels are attained; however, the contamination of the catalysts by the waste gases loaded with particles coming from the glass melting furnace poses problems of clogging and corrosion. After a certain length of time, the catalysts have to be replaced at considerable cost.